Photonic integrated circuits (PICs) generally integrate multiple photonic devices and respective photonic functions in the same chip, e.g., within the device layer(s) of a silicon on insulator (SOI) substrate with bonded III-V material. One or more of the photonic devices may include metal transmission lines disposed on top of the substrate. These transmission lines, which generally operate in the radio-frequency (RF) regime, can in principle carry large amounts of data at high speeds, data transmission being subject, however, to propagation loss due, at least in part, to substrate losses.
FIG. 1 depicts, in cross-sectional view, a portion of an SOI substrate 100 with an RF transmission line 102 disposed thereon, illustrating the basic layer structure of the substrate 100 and the origin of RF losses therein. As shown, the SOI substrate 100 includes a silicon handle 104, a buried oxide (BOX) layer 106 (e.g., made substantially of silicon dioxide) thereabove, a silicon device layer 108 on top of the BOX layer 106, and, optionally, a cladding layer 110 (e.g., made of a dielectric material) above the silicon device layer 108. (In the absence of a separate cladding layer 110, air serves as the cladding.) Photonic devices (not shown) may be embedded in the silicon device layer 108 and/or the cladding layer 110. The transmission line 102 generally includes multiple metal traces (e.g., two, as shown) that function as electrodes. As an RF electrical current is applied to these electrodes, an RF electrical field that extends through the SOI substrate 100 is formed, as indicated by the field lines 112. Parasitic conductive layers 114 formed at the interfaces of the BOX layer 106 with the silicon handle 104 and with the silicon device layer 108 allow for electric currents to flow in response to the electric field, reducing its field strength and thereby contributing to the RF losses in the substrate 100; substantial losses may occur, for example, in the frequency range from about 0.1 GHz to 40 GHz or even higher. In order to enable lower propagation loss in the transmission line 102, it is desirable to find ways of reducing these substrate losses.